Syrian prison authorities at the infamous Saydnaya Military Prison near Damascus called it "the party", a sick euphemism for an alleged crime against humanity.

This was the name for the weekly hangings held at the building dubbed "the human slaughterhouse", according to Amnesty International.

Victims would be collected from their cells and transferred to the basement of a red building, where they'd be severely beaten for hours.

Then, in the middle of the night, they were blindfolded and taken to a white building.

There, they were told they'd been sentenced to death, and within minutes, they were hanged alongside up to 50 other prisoners.

The rights group estimates up to 13,000 people died like this over four years to 2015, in a process authorised by "officials at the highest levels of government".

The Syrian Observatory for Human Rights puts the figure closer to 30,000.

So many were killed that satellite imagery showed what's believed to be a new crematorium built in 2017 to dispose of bodies.

Former inmates also described a series of "salt rooms" — makeshift morgues to preserve bodies in lieu of refrigeration.

On top of the death toll, many thousands more are believed to have been tortured, starved, and raped with "the apparent goal to humiliate, degrade, dehumanise and to destroy any sense of dignity or hope".

Prisons like this were central to ousted President Bashar al-Assad's ability to crush a civilian uprising, and they're currently the centre of a desperate search by families to find their missing loved ones.

Now, the world is beginning to hear from the survivors and demand justice for those who went in and never came out.

The frantic search for survivors

As Syrian rebels led by Hayat Tahrir al-Sham captured Damascus, they also threw open the doors of the Assad regime's notorious prisons.

The former dictator set up an industrial-scale system of arbitrary arrests and detention, where dissent was crushed in prisons that became synonymous with torture.

At the weekend, as his regime was overthrown, celebrations were followed by desperation, as families raced to the prisons where an estimated 130,000 people had disappeared during the 13-year civil war.

Relatives frantically searched through stacks of printed prison records, crowded into tiny windowless cells looking for familiar faces, and broke down walls in the vast underground complex of Saydnaya Prison.

They were searching for relatives that went "behind the sun," the Syrian term for being taken by the country's repressive security services.

And from the darkness, prisoners walked free; dazed, terrified, frail and unable to comprehend that Assad was actually gone.

Dr Mahmoud Mustafa from the Independent Doctors Association went into the prison and was shocked by what he saw.

"This prison has terrified people. On top of a mountain, so huge. We saw on people's faces how terrorised everyone was," he said.

He's now treating freed prisoners at a mosque nearby as many are gravely ill and don't know how to find their relatives.

"Most of detainees have various medical problems," he said.

"They have developed chronic diseases due to bad medical care in the cells, it was scary. No human being should live in these conditions."

Ammar Asalmo from the Syrian civil defence group, the White Helmets, agreed.

"It wasn't made for a human because you can see dozens of people in such a tiny place," he told the ABC.

"There was no hygiene, it's miserable."

He said it was a time of both celebration and grief for so many Syrians.

"All the families I know were crying because they celebrate the victory, but also, they're missing their people," he said.

"Because [they have gone] maybe 10 years without knowing anything about them. Even my wife, two of her uncles are considered missing. They were taken by the regime."

The group is also investigating reports from survivors of a network of hidden underground cells at Saydnaya.